1. Field of the Invention
This invention relates to a projection exposure method and a projection exposure apparatus suitable for use, for example, in a lithography process for the manufacture of semiconductors.
2. Related Background Art
For example, in the process of manufacturing a semiconductive element or a liquid crystal substrate by the use of lithography technique, use is made of a projection exposure apparatus for transferring a reticle pattern such as a circuit pattern onto a substrate at a predetermined magnification through a projection optical system. In such a projection exposure apparatus, the line width of the pattern to be transferred is as minute as e.g. 0.5 .mu.m. To image a pattern of such minute line width well, it is necessary to make the numerical aperture (NA) of the projection optical system great. Also, as a reticle, use has recently be made of a so-called phase shift reticle or the like which positively makes the most of the interference effect of light, but for such a phase shift reticle, it is necessary to enhance the coherency of exposure light.
In FIG. 6 of the accompanying drawings which is a schematic view of the illuminating system of a projection exposure apparatus according to the prior art, exposure light emitted from a fly-eye lens 1 as a secondary light source and passed through the opening portion of a variable aperture stop 2 disposed near the exit surface thereof (the focal plane adjacent to the reticle side) is converted into a substantially parallel light beam by an output lens 3. Of this substantially parallel light beam, a light beam passed through the opening portion of a reticle blind 4 as a variable field stop is converted into substantially parallel exposure light IL of large aperture by a relay lens 5 and a main condenser lens 6 and illuminates a reticle 7 with substantially uniform illuminance. The pattern forming surface of the reticle 7 and the opening portion of the reticle blind 4 are in an optically conjugate positional relation, and by changing the shape of the opening portion of the reticle blind 4, the shape and size of the illuminated area on the reticle 7 can be set arbitrarily.
The pattern on the pattern forming surface of the reticle 7 is transferred onto the exposure surface of a wafer 9 at a predetermined magnification by a projection lens 8. A variable aperture stop 10 is disposed on the pupil plane P1 of the projection lens 8, and the variable aperture stop 2 and the variable aperture stop 10 are in an optically conjugate positional relation.
As numerical values representative of the characteristic of the illuminating system of the projection exposure apparatus, use is generally made of the numerical aperture NA of the projection lens and .sigma. value representative of the coherency of exposure light. Describing the numerical aperture and .sigma. value with reference to FIG. 6, the maximum angle .theta..sub.R at which the light beam from the reticle 7 side of the projection lens 8 can pass and the maximum angle .theta..sub.W of the light beam falling from the projection lens 8 to the wafer 9 side are limited to predetermined values by the variable aperture stop 10 on the pupil plane P1 of the projection lens 8. The numerical aperture NA.sub.PL of the projection lens 8 is sin .theta..sub.W, and is in the relation that sin .theta..sub.R =sin .theta..sub.W /m when the projection magnification is 1/m.
Also, when the maximum angle of incidence at which the exposure light IL is incident on the reticle 7 is .theta..sub.IL, .sigma..sub.IL which is the .sigma. value of the illuminating system producing the exposure light IL is defined as follows: EQU .sigma..sub.IL =sin .theta..sub.IL /sin .theta..sub.R =m.multidot.sin .theta..sub.IL /sin .theta..sub.W.
Generally, the greater is the numerical aperture NA, the more is improved resolution, but the depth of focus becomes shallow. On the other hand, the smaller is the .sigma. value, the better becomes the coherency of the exposure light IL and therefore, when the .sigma. value becomes small, the edge of the pattern is emphasized, and when the .sigma. value is great, the edge of the pattern becomes blurred. Accordingly, the imaging characteristic of the pattern is substantially determined by the numerical aperture NA and the .sigma. value. Also, when the .sigma. value is varied by the variable aperture stop 2, the illuminance distribution on the pupil plane P1 of the projection lens 8 is varied.
The sequence when in the prior-art projection exposure apparatus of FIG. 6, a reticle pattern is exposed on a wafer will hereinafter be described with reference to FIG. 7 of the accompanying drawings.
In this case, first at a step 101, the operator manually operates the variable aperture stop 10 of the projection lens 8 to thereby set the numerical aperture NA of the projection lens 8 to a value that can resolve the minimum line width on the reticle to be transferred. The operator then operates the variable aperture stop 2 to thereby set the .sigma. value of the illuminating system in accordance with the reticle 7 to be transferred, and thereafter sets the reticle to be transferred on a reticle holder (step 102).
Thereafter, the mask pattern of the reticle is transferred to a plurality of wafers in succession through the projection lens 8 (step 103). When the optimum numerical aperture NA of the next reticle to be transferred and the .sigma. value of the illuminating system differ, at a step 104, the operator manually adjusts the variable aperture stop 10 and the variable aperture stop 2. Thereafter, the operator exchanges the reticle (step 105), whereafter the exposure process for a plurality of wafers is again carried out (step 106).
As described above, in the prior-art projection exposure apparatus, the two parameters, i.e., the numerical aperture NA and the a value, are apparatus constants and therefore, once the operator manually effects setting, it has been impossible to change those two parameters in a series of exposure sequences. However, the optimum numerical aperture NA and .sigma. value differ depending on the required imaging performance such as the minimum line width of the projection-exposed pattern on the reticle or the dimensional fidelity of the transferred pattern on the wafer to the mask pattern on the reticle.
Accordingly, where an IC for so-called specific use (ASIC) is to be manufactured and the patterns of plural kinds of device chips differing in the value of the optimum imaging parameter are to be exposed on the same wafer, it has heretofore been impossible to change the value of the parameter in the course of the process and therefore, it has been unavoidable to set the value of the parameter to the optimum value for only a particular one of those plural kinds of patterns. This means that the other patterns have not always been exposed under optimum imaging conditions.
Likewise, where a plurality of patterns differing in the value of the optimum parameter exist in the reticle, it has heretofore been unavoidable to set the value of the parameter to the optimum value for only a particular one of those patterns.